Unequal Cost Load Balancing (UCLB)
If there are multiple paths to the same destination with equal costs, it is reasonable to use
them all and share traffic equally among the paths. Alternate paths are guaranteed to be
loop-free, as they are “symmetric” with respect to cost to the primary path. If we there are
multiple paths of unequal cost, the same idea could not be applied easily.
In unequal cost load balancing, amount of traffic send across a particular path is inversely
proportional to the path’s metric.
Calculate Variance for the network given below:
If you are using only delay to calculate the metric, what should be the value of variance to
allow unequal cost load balancing to reach R2’s loopback address from R1, if EIGRP is
configured as the routing protocol?
To calculate variance use the formula given below:
metric < best_metric*variance
metric: is the full metric of the alternate path and
best_metric: is the metric of the primary path.
Normally variance value is 1, meaning that only equal-cost paths are used
The best way to implement UCLB in real life is by using MPLS Traffic Engineering. This
solution allows for high level of administrative control, and is IGP agnostic (well they say you
need OSPF or ISIS, but you may use verbatim MPLS TE tunnels even with RIP or EIGRP). It is
safe to use unequal-cost load-balancing with MPLS TE tunnels because they connect two
nodes using “virtual circuits” and no transit node ever performs routing lookup